Traditionally, the performance evaluation of voice transmission systems in telephony is based on measurements using sinusoidal input signals. Typically, an overall sensitivity/frequency response of a telephone connection (from talker's mouth to listener's ear), either measured directly, or calculated from the responses of the individual parts of the connection, is used to evaluate the loudness level perceived by the listener, the effective bandwidth affecting the intelligibility of transmitted speech, etc. Complex models, based on subjective tests, are then used to combine such attributes of statistically sampled connections to evaluate the effects of introducing new devices into the telephone network in order to maintain or improve grade of service and achieve system economics.
The characterization of a voice transmission system by means of a sinusoidal input signal is strictly valid only for linear systems. However, most telephone connections involve at least one high non-linear element--the carbon microphone. Significant discrepancies are observed on telephone sets with carbon microphones between the expected performance derived from measurements with sinusoidal signals and those experienced in subjective tests using real voice. This is described in a paper entitled "Comparable Tests on Linear- and Carbon-Type Microphones" by H. W. Bryant, The Journal of the Acoustic Society of America, Vol. 53, No. 3, 1973, pp 695 to 698. Such discrepancies in expected performance are not nearly as apparent for sets with linear microphones. Additionally, it has been found that close agreement between measured and subjective tests can be realized for non-linear systems if the signal used for their characterization approximates the relevant properties of real voice.
In order to approximate real voice, the traditional single frequency test signal must be replaced by a wideband signal with a power spectrum density similar to that of an average speech signal. If only frequency response measurements of carbon microphones were required, then the exact shaping of the spectrum of such a test signal does not appear to be critical. Quite satisfactory results, i.e. results in agreement with real voice measurements have been obtained using pink noise. However, for wider applications, e.g. for measurements of signa/distortion ratio, this technique does not yield satisfactory results.